A pipette is typically used in laboratories for measuring and transferring small volumes of liquids. When the pipette volume must be known accurately, it is necessary to calibrate the volume of the pipette. A number of prior art techniques are known for volume calibration of pipettes.
In a prior art gravimetric technique, water is dispensed from a pipette being calibrated onto a sensitive balance. The water is weighed and the volume is calculated based on the weight of the water. Although this technique can provide accurate measurement of pipette volume, it is relatively expensive and complex when performed properly. For small volume pipettes, the required balance is relatively expensive. Furthermore, the accuracy is sensitive to evaporation of the water, the temperature of the water, the ambient barometric pressure, static electricity, vibration and ambient air movement.
In a prior art colormetric technique for calibration of pipette volume, a cuvette is provided with a predetermined volume of water or other solvent. A liquid containing a single dye is transferred from a pipette being calibrated into the cuvette and mixed. The cuvette is then positioned in the spectrophotometer and the absorbance of the liquid is measured. The volume of the pipette is calculated from the absorbance of the liquid in the cuvette. This technique is subject to inaccuracies, including optical variations and temperature variations. Typically, one aliquot of dye solution is added to the water in the cuvette and a measurement is taken. Then, the liquid in the cuvette is discarded and the same process is repeated several times to insure repeatability. This process is relatively inaccurate and time consuming. In particular, the variation in pathlength between each cuvette leads to inaccuracies, as well as the variation in pathlength when an individual cuvette is repositioned in the photometer between readings.
In a third prior art technique, the pipette being calibrated is used to transfer an acid solution into a container. Then a mechanical burette is used to add a base solution containing an indicator dye to the container. The indicator dye changes color when the concentration of acid equals the concentration of base. At this point, the volume of base added is read from the mechanical burette. This technique is relatively difficult and time consuming.
It is a general object of the present invention to provide improved methods and apparatus for calibrating the volume of a pipette or other vessel.
It is another object of the present invention to provide a reagent system for determining the volume of a pipette utilizing a photometer.
It is a further object of the present invention to provide methods and apparatus for determining the volume of a pipette with high accuracy.
It is still another object of the present invention to provide methods and apparatus for determining the volume of a pipette with a photometer which do not require a highly accurate container for measurement.
It is yet another object of the present invention to provide methods and apparatus for determining the volume of a pipette which are easy to use and low in cost.